Because of their high efficiency, conventional multijunction solar cells have been widely used for terrestrial and space applications. Multijunction solar cells include multiple diodes in series connection, known in the art as “junctions,” realized by growing thin regions of epitaxy in stacks on semiconductor substrates. Each junction in a stack is optimized for absorbing a different portion of the solar spectrum, thereby improving efficiency of solar energy conversion.
Conventional multijunction solar cells have features that reduce the efficiency of solar to electrical energy conversion. For example, a portion of solar energy incident on the front side of a solar cell cannot be absorbed due to metallic electrodes blocking a portion of the side facing the sun. Furthermore, a portion of the absorbed solar energy cannot be collected at the electrodes as electrical power because the energy is dissipated as heat (for example, as resistive loss) during lateral conduction in the emitter region of the top junction and in the metallic gridlines. For high-power devices, such as concentrated photovoltaic devices or large area solar cells, the dissipated heat may also result in substantially increased temperature, thereby further reducing the performance of the device. Typically there is a trade-off between these parameters and others. Multijunction solar cells are typically designed to give the optimum solar to electrical energy conversion performance under desired conditions. It is desirable to improve efficiency in multijunction solar cell devices.
Previous solar cells with weak-current-producing layers suffer from low current and low efficiency. When incorporated in a multijunction cell, the low current production of one of these subcells may limit the current of the entire series-connected multijunction solar cell stack. With the low-current solar cell/reflector structures in the various aspects of the present disclosure, significantly higher currents can be achieved in solar cells with low absorption coefficient or low minority-carrier collection probability, resulting in higher efficiency, more cost effective photovoltaic cells.